1. Field of the Invention
The present invention relates to a coding method and, more particularly, relates to a redundancy reduction coding method capable of largely reducing an amount of data required for transmission or storage of multi-tone picture.
2. Description of the Prior Art
Conventionally, this kind of coding method includes a linear predictive coding method. FIG. 1 is a drawing showing an objective picture element X.sub.0 to be transmitted and reference picture elements X.sub.1 -X.sub.3 selected from picture elements already coded and transmitted. In the figure, A denotes a main scanning direction and B denotes a sub scanning direction. Let it be assumed that the tone levels of the picture elements in FIG. 1 are x.sub.0, x.sub.1, x.sub.2 and x.sub.3, respectively, a predictive value x.sub.0 of the objective picture element of X.sub.0 provided by a linear predictive function in the following equation (1), for example, has been well known to provide a relatively excellent prediction. ##EQU1##
In the linear predictive coding method, a difference value (referred to as a predictive error) between the predictive value x.sub.0 of the objective picture element evaluated using the above described predictive function and an actual value x.sub.0 of the objective picture element is evaluated and the evaluated difference value is coded and transmitted. As a matter of course, a pattern of distribution of the difference values indicates a mountain-shape having a peak at 0. Accordingly, data compression becomes possible by performing a coding in which a short code word is assigned with respect to a difference value near 0 using a variable length code.
Meanwhile, in a conventional linear predictive coding method, a detailed statistic nature of a picture is not necessarily and fully considered and hence there is some possibility that efficiency of coding is reduced depending on a situation. For example, let it be assumed that in a picture of 32 tones, the tone levels of the reference picture elements X.sub.1 -X.sub.3 as shown in FIG. 2 are 6, 1 and 3, respectively. At that time, the distribution of the tone level x.sub.0 of the objective picture element is considered a two-peak type having two peaks at 1 and 6. More particularly, the most likely value of x.sub.0 is 1 or 6. On the other hand, in this case, the predictive value x.sub.0 evaluated from the linear predictive function of the equation (1) is 4. Thus, a predictive error becomes larger and hence an effective redundancy reduction coding cannot be made.